Wind turbines are often deployed in remote locations such as on hilltops or offshore. As such, gaining access to the wind turbines for repair purposes is typically expensive. To mitigate the risk of having to repair wind turbines early in their working lifetime, it is necessary to rigorously test the components of each wind turbine before installation. One such component is the gearbox used to convert relatively low-speed, high torque movements of the turbine blades to the higher-speed, lower torque movements required by an electricity generator.
Gearboxes intended for use in wind turbines typically require dedicated test rigs. The simplest form of test rig is an open loop test rig comprising a drive motor for applying a low-speed, high torque signal to a low-speed shaft of the gearbox being tested to simulate the movement of the wind turbine blades. Measurements can be performed on the low-speed and corresponding high-speed shafts of the gearbox to test its performance. However, the energy input by the drive motor must be dissipated constantly by means of, for example, a braking device which must be kept cool. As such, open loop test rigs are inefficient.
A more efficient, closed-loop gearbox is described in DE19704016. In this case, the respective low and high speed shafts of two gearboxes are coupled to each other, via additional gearboxes. One of the said two gearboxes is a summing gearbox comprising a third shaft to which the main load torque is applied. An electric motor is also provided to overcome losses due to mechanical friction in the system. By coupling together the respective shafts of the gearboxes, a closed-loop mechanical circuit is formed whereby energy input to the test rig can be recycled. However, the “third shaft” of the summing gearbox is typically the casing of the summing gearbox. Rotating the casing of the gearbox would typically be impractical and inefficient, particularly for a heavy duty wind turbine gearbox, due to the large amount of energy which would be required. In addition, the high energy requirements would make it difficult to test the gearbox for transient loads.
A further restriction of the test rig of DE19704016 is that the overall transmission ratio of all the gearboxes on the path from the output shaft to the input shaft of the gearbox being tested must be equal to that of the gearbox being tested.
It would thus be desirable to provide a new gearbox test rig design which overcomes the above mentioned problems.